1. Field of the Invention
This invention relates to electrophotography, and more particularly to an improved method and apparatus for controlling the level of electrostatic charge on a surface upon which an electrophotographic image is to be made.
2. Description of the Prior Art
In known electrophotographic reproduction apparatus such as copiers or duplicators, an electrostatic charge is deposited on an area of a photoconductor as the area is moved past a charging station. The photoconductor is then moved to an exposure station where the area is exposed to image-forming radiation to form a latent electrostatic image of a document to be copied. The latent image is thereafter developed and, in the case of plain-paper copiers and duplicators, subsequently transferred to paper upon which the copied image is to appear. Thereafter, the photoconductor is cleaned and otherwise made ready for the next copy cycle.
In such apparatus, it is important to impart a generally uniform charge over the area upon which the latent image is to be formed. Too low a charge in portions of the area may result in weak, washed-out looking areas on copies, and too great a charge in portions of the area may result in areas on copies being too dark relative to other areas. Therefore, copy quality, particularly with pictorial subject matter, is affected seriously where a non-uniform charge is placed on the photoconductor.
In the prior art, as exemplified by U.S. Pat. No. 4,105,321, an apparatus is described for controlling the level of charge placed on a photoconductor. In this apparatus a corona wire is energized to deposit a positive electrostatic charge on a photoconductive belt. This level of charge imparted to the belt is measured by an electrometer that is located proximate the belt and a signal from the electrometer is compared with a reference signal that represents a maximum level to which it is desired to charge the belt. If the signal derived from the electrometer indicates that the charge is beyond this maximum level, an erase light source is illuminated to the brightness necessary to reduce the level of charge deposited on the belt.
In electrophotography, it is known to charge a photoconductor with either a negative or a positive electrostatic charge, the particular charge chosen being a selection based on the type of photoconductor used. This charge is preferably deposited by a corona charging system comprising one or more corona wires which lie transverse to the direction of movement of the photoconductor past the charging station. In the case of employing positive corona charging, a generally uniform charge can be expected to be deposited on the photoconductor using known charging techniques. In the case of negative corona charging, control is more difficult, and a considerable amount of charge non-uniformity can result. One may observe the glow from a positively charged corona wire and note a uniform glow surrounding the entire length of the wire to indicate uniform current flow from the wire to the photoconductor. On the other hand, discrete glow spots often occur along a negatively charged corona wire. The glow spots are associated with creating non-uniformities in charging of the photoconductor. As the glow spots appear at different positions along the wire over the course of a day due to changes in humidity within the copier or because of other factors, the non-uniformity of charging also changes with time (see R. M. Schaffert, Electrophotography, 1975 edition, pages 466-472). While minor amounts of non-uniformity may be tolerated, significant non-uniformity as indicated above represents a problem. The extent of the problem will depend upon the nature of the apparatus and the material to be copied. Obviously, continuous tone or halftone originals will be more of a problem than copying text. Color copiers demand even more uniformity in charging than do monotone copiers.
It would therefore be very desirable to have a method and apparatus for providing over the image-forming area of a photoconductor a generally uniform charge, particularly where because of the charging source used or because of other conditions there is a tendency for a non-uniform charge to otherwise be deposited on the surface in a direction transverse to the direction of movement of the surface past the charging source. As used herein, the term, "generally uniform charge," implies that variation in charge level may occur from one part of the area to another; however, the various parts comprising the area have a level of charge falling within a desired narrow range of values.